1. Technical Field
The invention relates generally to semiconductor device fabrication, and more particularly, to methods of constructing a selective semiconductor-on-insulator (SOI) structure that has self-aligning attributes amongst its various parts, and a related structure.
2. Background Art
Semiconductor-on-insulator (SOI) substrate wafers (or, “SOI structure”) are a known improvement over conventional bulk wafers for both solving the problem of electrical isolation between adjacent devices and also allowing for more complicated layout designs. Due to its reduced junction capacitance, a metal-oxide-field effect transistor (MOSFET) structure employing the SOI structure also results in improved circuit switching speed. However, the SOI structure also has a drawback of lacking any body contact, which results in several adverse effects, such as the floating body effect and the history effect. The floating body effect allows a dynamic lowering of the threshold voltage which may create a history effect which includes a propagation delay of a gate, depending on what happened to the gate beforehand. A SOI structure typically includes a thin layer of silicon oxide (“oxide layer”) inserted into, and thus buried within, a silicon substrate.
A species of SOI construction that is gaining favor is selective SOI which may include the selective formation of the buried oxide layer, wherein the buried oxide layer is not necessarily continuous and uninterrupted laterally along the substrate as with “traditional” SOI construction, hence the term “selective”. By inserting the buried oxide layer only underneath the source/drain area, the selective SOI structure can maintain the benefits of a traditional SOI structure, while eliminating the drawbacks of a traditional SOI structure that are due to the lack of a body contact.
However, as with traditional SOI construction, selective SOI suffers from various shortcomings. For example, with the ongoing scaling down of dimensions in fabrication (e.g., ever smaller channel length, etc.), it is increasingly difficult to properly align the selective SOI structure only in the source/drain region. This misalignment, or lack of self-alignment, of the various elements within a selective SOI structure is undesirable. Performance may be degraded and device-to-device variation may increase as a result. Another shortcoming is that the formation of gate structures and adjacent implanted oxide layers are typically imperfect. This problem arises regardless of whether the buried oxide layer is inserted via the separation by implantation of oxygen (SIMOX) method, or via other available methods.
In view of the foregoing, there is a need in the art for a solution to the problems of the related art.